WO2015097704A1

WO2015097704A1 - Abrasive surface preparation

Info

Publication number: WO2015097704A1
Application number: PCT/IL2014/051128
Authority: WO
Inventors: Pini LAHAV
Original assignee: Camel Grinding Wheels
Priority date: 2013-12-25
Filing date: 2014-12-25
Publication date: 2015-07-02

Abstract

Coated abrasive particles comprising: abrasive particles. a ceramic coating, the coating comprising: frit 60 to 90% w/w of the coating; feldspar 0 to 25% and clays 5 to 30%.

Description

ABRASIVE SURFACE PREPARATION

FIELD OF THE INVENTION

The present invention relates to enhancing the performance of abrasive articles such as cut-off discs.

BACKGROUND OF THE INVENTION

Abrasive grains are used in various sizes, shapes, compositions and surface modifications during the production of an abrasive product.

The common abrasive particles are typically made of different types of alumina, zirconia , silicon carbide and/or ceramics (sol-gel). In a mixture together with ceramic masses, artificial resin and other additives such as fillers, lubricants and colors, they are shaped into abrasive discs, with the ceramic masses or artificial resins serving as "binder phase" for the abrasive particles.

The strength of bonding between the abrasive particle and the binder phase is of great importance since the abrasive particles are subject to shear that acts to break the abrasive particles from the binder phase.

Some abrasive particles, in particular many of those that are produced by a conventional high temperature melting process have a smooth surface that proves to be disadvantageous for the bonding that relies primarily on adhesion. This holds true in particular for abrasives bonded by synthetic resins in which the binding is based almost exclusively on adhesion.

US Patent 7,381,466 describes treatment of abrasive particles for application in synthetic resin -bonded abrasives, in order to provide sheathed abrasive particles with an enlarged surface and improved bonding of the particles to the binder phase.

In the treatment, abrasive particles such as fused or sintered corundum, zirconium corundum, silicon corundum, silicon carbides and boron carbide are coated with 0.05 to 2.0 weight percent, relative to the weight of the untreated abrasive particle, of aqueous silicon based binding agents (silane coupling agents) and sheathed with 0.05 to 5.0 weight percent, also relative to the weight of the untreated abrasive particle, of fine grained oxides of metal and amphoteric elements.

The coated abrasive particles are subjected to a heat treatment between 100 and 900°C as part of the coating process. The oxides, of the formula A_xB_yO_z, are purported to adhere to the surfaces of the abrasive particles and thus increase the abrasive surface.

However, such coating of the abrasive grains does not appear to address several factors which adversely affect the performance of the abrasive product, such as poor heat transfer from the abrasive particles to the binder phase, and friability of abrasive particles that have faults therein as a side effect resulting from the manufacture of the abrasive particles.

Moreover, as result of routine use of the abrasive articles, new surfaces are exposed on the abrasive particle, following contact of the abrasive particle with the work piece. The oxide coating might thus be lost after the first contact and a drastic reduction in performance can subsequently occur.

Increasing production costs drive ever-increasing improvements in this field, in order to reduce abrasive grain costs and improve the performance of lower quality abrasives.

One objective of the invention is to provide an improved coating technology for abrasive particles, with thin coatings that improve heat transfer and adhesion of the particles, thus leading to significant improvement of the abrasion performance.

Another objective of the present invention is to improve the performance of cutting resinoid bonded grinding wheels containing abrasive grains made with materials such as sol-gel (ceramic) alumina and zirconia. SUMMARY OF THE INVENTION

According to one aspect, coated abrasive particles are provided, the coated particles comprising:

abrasive particles;

a ceramic coating, the coating comprising:

frit 60 to 90% w/w of the coating;

feldspar 0 to 25%,

and clays 5 to 30%.

In some embodiments, the abrasive particles comprise zirconia, and the frit has a sintering temperature lower than 540°. the abrasive particles may comprise one or more particles of the group consisting of: synthetic corundum, zirconia, silicon carbide, CBN, diamonds , an alumina selected from: White Alumina (WA), Brown Alumina (BA), Semi-friable Alumina (SA), Blue Fired

Alumina (BFA), Pink Alumina (PA), Ruby Alumina (RA) and mixtures thereof, and zirconia- alumina ZA40 and ZA25.

The coated abrasive particles are preferably smaller than grit 120 but over 40 mesh.

According to another aspect, a method for coating abrasive particles is provided, the method comprising:

making a mixture by mixing abrasive particles, a combustible composition and a ceramic composition; heating the mixture until the combustible composition is essentially removed.

When the abrasive particles comprise ZA40, the heating is to a temperature of between 350 and 550°C.

The ceramic composition: combustible composition ratio before heating is typically 0.5 to 5.0: 1 w/w.

Preferably, the ceramic composition: combustible composition ratio before heating is 2.0 to 3.0 w/w; more preferably between 2.3 and 2.7; even more preferably between 2.4 and 2.6.

In some embodiments the coating comprises:

frit 60 to 90% w/w of the coating;

feldspar 0 to 25%,

and clays 5 to 30%,

and the heating is to a temperature below the sintering temperature of the frit.

In some embodiments the method further comprises:

allowing the heated mixture to cool to room temperature;

sieving the mixture through a #40 sieve, and

collecting particles in the mixture that are coarser than #40 mesh. In some preferred embodiments the method further comprises allowing the heated mixture to dry for a day.

The combustible material is selected from a group consisting of: carbohydrates, waxes, liquid phenolic resols, novolaks based on phenolics, modified phenolics, polyesters, epoxies and combinations thereof.

Very good results were obtained when the combustible material is dextrin.

According to another aspect, the coated abrasive particles (prepared by any of the methods and their embodiments indicated above) are provided.

According to another aspect, a cutting disc comprising coated abrasive particles described above is provided.

According to yet another aspect the abrasive particles are made of sol-gel alumina and the ceramic coating is defined by Table 1.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a schematic drawing of abrasive particles coated with the ceramic coating and bonded together, to make a composition that may be attached onto a cutting disc (the disc is not shown);

Figure 2 shows a micrograph of an untreated SG36 particle; Figure 3 shows a micrograph of a treated SG36 particle;

Figure 4 shows a micrograph of an untreated ZA40 particle; Figure 5 shows a micrograph of a treated ZA40 particle;

Figure 6 shows an electron micrograph of a coated ZA40 grit, at 1000X magnification;

Figure 7 shows a micrograph of an untreated WA particle (99.6% alumina and about 0.25% Na₂0) and

Figure 8 shows a micrograph of a treated particle.

DETAILED DESCRIPTION OF THE INVENTION

It is an objective of the present invention to provide improved cutting discs by preparing novel modified abrasive grains. The grains may include for example sol-gel ceramic and/or alumina- zirconia particles.

The abrasive particles are indirectly bonded to each other on the cutting discs. The abrasive particles are coupled to each other by means of a phenolic resin or other organic matrices such as epoxies and polyesters, or any other cross linking resins available in the industry for the preparation of cut-off wheels.

Furthermore, some of the particles are coated with a ceramic coating, as will be further explained below. Figure 1 shows a schematic drawing of abrasive particles coated with the ceramic coating and bonded together, to make a composition that may be attached onto a cutting disc (the disc is not shown).

The composition 100 includes the abrasive grains 110, ceramic coating 120, a bonding phase 130 that includes bonding material 132 such as phenolic based resin, and a filler 134 (at a ratio of roughly 2: 1 of bonding material to filler). In between the grains there are often voids 140.

The abrasive particles may be alumina driven grains, such as ceramic grains, White Alumina (WA), Brown Alumina (BA), Semi- friable Alumina (SA), Blue Fired Alumina (BFA) and sintered rods, Pink Alumina (PA) and Ruby Alumina (RA), as well as zirconia- alumina grains (both ZA40 and ZA25) in various sizes, up to grit 120.

The modified (coated) particles include for example particles made of abrasive ceramic grains manufactured by the sol-gel method at temperatures of -960° C, i.e. below fusion temperatures, and/or by preparation of a eutectic mixture ZA40 (40% zirconia and 60% alumina), although in some embodiments modified ceramic grains and zirconia mixtures made by other preparation methods are included instead or in addition to the aforementioned grains. Such modified particles may be at a concentration of only about 10% w/w or even less of the total amount of grains in the cutting discs. These particles are covered by a ceramic coating as part of a novel process that is explained below.

The cutting discs made from the improved coated particles typically have superior performance attributed to the following properties: a) Superior adhesion of the ceramic coating to the abrasive grain;

b) Excellent adhesion of the ceramic coated abrasive particles to the bond system (resin);

c) Chemical inertness of the ceramic coating;

d) Repair of faults in the abrasive grains by their coating;

e) Control of the cutting surface roughness;

f) The coating has an effect that is similar to that imparted by an active filler that contributes to heat dispersion of the grain while grinding and/ or cutting. The ceramic bond is endothermic and dissipates heat; and thus helps control the coolness of the cutting.

The improvement of the cutting discs is exhibited also by an increased cutting speed and longer life of the discs over commercially available discs, as will be shown below.

According to one aspect, a composition for preparation of improved abrasive particles is provided. The composition comprises: Abrasive particles containing materials such as sol-gel alumina, synthetic corundum, zirconia, silicon carbide, CBN, diamonds and combinations thereof;

at least one combustible compound, for example an organic compound such as a carbohydrate, wax, liquid phenolic resol, novolaks based on phenolics, modified phenolics, polyesters, epoxy and combinations thereof, and

a ceramic solid composition for preparation of the ceramic coating.

Note that "combustible" is used in the context of a property of a compound to undergo transformation upon heating that leaves behind no or little deposit; for example burning (reaction with air or self- reaction to produce gaseous products) and sublimation.

The process of preparing the improved abrasive particles involves mixing the above mentioned ingredients and subsequently firing, curing and expelling (baking) out the combustible compounds in a controlled heat profile aimed at completely removing the combustible material thereof.

In general, the abrasive particles are mixed with the

combustible material and with the ceramic solid coating material at high speed for a few minutes. The preferred speed varies according to the mixed materials and the type of mixer that is used. In some embodiments at least some of the combustible material is liquid or dissolved or suspended in a liquid. When producing modified abrasive particles from such embodiments, the liquid is stirred with the wet particles for about 5 minutes, i.e. usually 2-10 minutes, the preferred period of time again dependent upon the materials and the mixer, and subsequently the ceramic material and optionally the combustible solids are mixed in at high speed for about 5 minutes. The resultant mixture may be laid in baking trays to dry. The laying out to dry for about a day surprisingly may further improve the performance of the cutting disc.

In some embodiments at least some of the combustible compounds are cross linkable; in some embodiments a cross-linking composition is added to the mixture to cross-link the cross-linkable compounds; for example, the combustible compounds may comprise phenolic and/or epoxy resins in oligomeric form; alternatively, novolaks are provided in solid powder form and suspended in water, mixed to wet the abrasive particles, and then the same novolak in powder form may be added together with the ceramic material. In fact, phenolic based resins in various forms and a wide range of other organic compounds have been tested and found to improve the performance of cutting wheels made with the abrasive particles.

The ceramic composition of the coating is generally selected according to its melting point, which should not exceed the typical temperature developed while the cutting disc is used in its designated purpose (which in turn depends on the material the abrasive particles are made of). This temperature is generally about 700-800°C. The ceramic composition should allow good heat transfer from the abrasive particles to the ceramic coating which helps prevent excessive heating of the abrasive particles. This feature is particularly important when the abrasive particles comprise zirconia.

An example of ceramic composition range that has been found to be suitable for use with alumina abrasive particles is shown in Table 1.

Table 1

Component % weight

Li₂0 0-5%

Na₂0 5-10%

K₂0 0-5%

MgO 0-2%

CaO 0-4%

ZnO 0-4%

A1₂0₃ 0-25%

B₂0₃ 5-35%

Si0₂ 30-70% The composition of the ceramic coating is altered in order to control the adhesion of the coat to the abrasive particles, and its melting properties. While the Na₂0 content and B₂0₃ are important for lowering the melting temperature, the Li₂0 is important to improve the adhesion behavior and strength. Generally, different types of frits make up 60 to 90% of the weight of the ceramic coating, Feldspar 0 to 25%, and clays 5 to 30%. For abrasive particles comprising zirconia the frit that is selected should have a sintering temperature lower than 540°C.

One example of a combustible compound that was found to be a suitable additive is dextrin, which was provided both in the liquid at a concentration of 75% w/w in water, and in powder form. Selection of the combustible material may depend upon the abrasive particle type. For example, carbides are susceptible to oxidation and thus the combustible compound may be a polymer that helps scavenge oxidizing species and/or does not decompose into peroxides.

The ratio of the combustible material, liquid and ceramic composition that has been found to be optimal in respect of the cutting performance of the discs may vary; typically the approximate ratio of ceramic composition to net dry combustible material is about 5: 1 to 0.5: 1, more particularly closer to 3: 1, respectively. The optimal ratio depends upon the particular composition. For ZA40 particles the optimal ratio is about 2.5; i.e 2.0 to 3.0: 1, more preferably 2.3 to 2.7:1, even more preferably 2.4 to 2.6: 1. The heating conditions generally are reliant upon the

composition of the abrasive particles and the ceramic coating composition as well as the combustible agent. When the abrasive particles comprise ZA40, the final heating temperature will not exceed 600°C, in order to prevent adverse changes to the crystalline structure of the abrasive particles; the final temperature of the heating process is thus preferably between 350 and 550°C when the coated particles include zirconia. The minimum temperature is above the combustion temperature of the combustible material.

When the coated abrasive particles are alumina sol-gel, the temperature range is rather larger, between 400 and 900° C.

It is important to maximize removal of the combustible material during the firing process in order to optimize the cutting performance potential of the coated abrasive particles. For the abrasive particles comprising zirconia the rate of the heating process may be more expanded, whereas for the abrasive particles comprising sol-gel alumina without zirconia, higher rates of heating allow faster completion of removal. The heating process can last up to 20hrs. In some embodiments the heated coated particles are sifted through a sieve and the coarser particles, usually 40 mesh or coarser, are used for the manufacture of the disc.

It is stressed that not all of the abrasive particles in the cutting disc/abrasive wheel are coated with the coating procedure described above; rather only a small portion of abrasive particles are coated, yet they may exert a dramatic effect on the product's performances.

Discs which contain in their formulation the abrasive sol-gel alumina particles 3M™ Cubitron™ SG36 Abrasive Grains,

(http://solutions.3m.eom/wps/portal/3M/en_WW/Abrasive_Systems/H ome/Technologies/one/ (seeded gel ceramic grits containing a alumina and various oxides, sized mesh #36), are well known as excellent cutters); the Cubitron abrasive particles are thought to continually reveal fresh cutting edges on the surface of the work piece ensuring a sustained cut and longer product life. However, it was found that untreated Cubitron abrasive particles do not appreciably improve performance of cutting discs incorporating the particles.

Two grinding discs sized 4.5* l/8"*7/8" were prepared from untreated Cubitron abrasive particles and from a ceramic coated grain. The composition of the ceramic coat is based on a composition listed in Table 1.

Two other discs were prepared from the same Cubitron particles coated with the ceramic composition, and the dextrin as described above as indicated in Table 1.

Comparative performance results of a 3.2mm thickness grinding disc tested on a hardened steel plate are summarized in Table 2. Table 2

E is the erosion rate of the removed stock, gr/min; G is the ratio between the material lost from the grinding disc and the material lost from a specimen.

Discs made with the treated abrasive particles have a much superior cutting ability and dramatically reduced wear over discs made with untreated abrasive particles.

Figure 2 shows a micrograph of an untreated SG36 particle 10, Figure 3 shows a micrograph of a treated SG36 particle 210. Note the transparent ceramic coating. The coating does not cover the entire surface of the particle but rather is spread in drops 212 of various sizes over the surface of the particle 210.

The untreated SG particles apparently have smoother surfaces. This smoothness might cause a poor retainment of the uncoated SG particles by the cutting disk, thereby leading to the observed poor performance. The proposed ceramic coating beneficially changes the cutting surface's roughness.

The ceramic coating is preferably at least 0.25% of the weight of the abrasive particles. The coating is usually no more than 7% of the grain's weight. The combustible component in the preparation is usually up to 7% of the weight of the coated abrasive particles.

The abrasive Cubitron particles are very expensive; therefore their amount in cutting wheels is typically not more than 30%% w/w of the abrasive particles. Using the coating technology may provide an improved quality of the products as well as cost reduction by reducing the SG particle amounts.

Similarly, the very expensive eutectic zirconia mixture ZA40, when used in commercially available cutting discs, is usually present at a concentration of about 10-30% of the abrasive particles on the disc, and are primarily responsible for the superior cutting qualities of the disc. However, the amount of ZA40 may be reduced in some of the present embodiments to a mere 5% concentration, yet the discs retain the superior cutting properties.

Figure 4 shows a micrograph of an untreated ZA40 particle 20. Note the fissure 21 that weakens the particle and causes breakage of the particle when the cutting wheel is used. Figure 5 shows a micrograph of a treated ZA40 particle 220. Again, the ceramic coating does not cover the entire surface of the particle but rather is spread in drops 222 over the surface of the particle 220. The combustible material is essentially removed from the mixture by the baking out. Typically only residues may remain on some of the abrasive particles that underwent the mixing with the combustible material. The figure further shows residues 224 of dextrin on the surface of the particle 220. We have observed that the ceramic coating drops 222 tend to be situated next to the locations of the combustible material residue 224, as well as in fissures 221, implying that the combustible material has a beneficial role in determining the surface distribution of the ceramic material and repairing damages in the abrasive particles.

Figure 6 shows an electron micrograph of a coated ZA40grit 36, at 1000X magnification.

It is also thought that evacuating and expelling the combustible material out of the ceramic/combustible matrix that coats an abrasive particle beneficially creates a rougher exterior to the coated abrasive grain.

In some embodiments the provided uncoated abrasive particles alone have mediocre cutting performances, yet together with the proposed coating a disc may be manufactured with cutting parameters on par with the current best commercially available discs. For example, Figure 7 shows a micrograph of untreated WA (99.6% alumina and about 0.25% Na₂0) and Figure 8 shows a micrograph of a treated particle.

Note that there is some marked improvement of the performance of cutting discs when some of the abrasive particles are coated with the ceramic coating alone; however, the improvement may not be at all as large as that achieved when the coating preparation includes

combustible material that is baked out, as indeed happens when using the ceramic coating listed in Table 1 , together with dextrin as described above.

The ceramic coating thus affords several benefits:

1) the uncovered region on the abrasive particle creates a better interface with the bonding resin ;

2) the coated particle has improved cutting performance, and

3) the ceramic coating accelerates heat dissipation

It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub combination. Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives,

modifications and variations that fall within the spirit and broad scope of the appended claims.

Claims

1. Coated abrasive particles comprising:

abrasive particles;

a ceramic coating, the coating comprising:

frit 60 to 90% w/w of the coating;

feldspar 0 to 25%,

and clays 5 to 30%.

2. The coated abrasive particles of claim 1, wherein the abrasive particles comprise zirconia, and wherein the frit has a sintering temperature lower than 540°.

3. The coated abrasive particles of claim 1, wherein the abrasive particles comprise one or more particles of the group consisting of: synthetic corundum, zirconia, silicon carbide, CBN, diamonds , an alumina selected from: White Alumina (WA), Brown Alumina (BA), Semi- friable Alumina (SA), Blue Fired Alumina (BFA), Pink Alumina (PA), Ruby Alumina (RA) and mixtures thereof, and zirconia- alumina ZA40 and ZA25.

4. The coated abrasive particles of any one of claims 1 to 3, smaller than grit 120.

5. The abrasive particles of claim 2, sized over 40 mesh.

6. A method for coating abrasive particles comprising: making a mixture by mixing abrasive particles, a combustible composition and a ceramic composition;

heating the mixture until the combustible composition is essentially removed.

7. The method of claim 6, wherein the abrasive particles comprise ZA40, and the heating is to a temperature of between 350 and 550°C.

8. The method of claim 6, wherein the ceramic composition: combustible composition ratio before heating is 0.5 to 5.0:1 w/w.

9. The method of claim 7, wherein the ceramic composition: combustible composition ratio before heating is 2.0 to 3.0 w/w.

10. The method of claim 9 wherein the ratio is between 2.3 and 2.7.

1 l.The method of claim 10, wherein the ratio is between 2.4 and 2.6.

12. The method of claim 6 or 7, wherein the coating comprises:

frit 60 to 90% w/w of the coating;

feldspar 0 to 25%,

and clays 5 to 30%, and wherein the heating is to a temperature below the sintering temperature of the frit.

13. The method of claim 6 or 7, further comprising:

allowing the heated mixture to cool to room temperature;

sieving the mixture through a #40 sieve, and collecting particles in the mixture that are coarser than #40 mesh.

14. The method of claim 6 or 7, further comprising allowing the heated mixture to dry for a day.

15. The method of claim 6 or 7, wherein the combustible material is selected from a group consisting of: carbohydrates, waxes, liquid phenolic resols, novolaks based on phenolics, modified phenolics, polyesters, epoxies and combinations thereof.

16. The method of claim 15, wherein the combustible material is dextrin.

17. Coated abrasive particles prepared by method 6 or 7.

18. A cutting disc comprising coated abrasive particles of any one of claims 1, 2, 3, 5 or 17.

19. The particles of claim 3, wherein the abrasive particles are made of sol-gel alumina and the ceramic coating is defined by Table 1.